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Revenue Recognition (Customer Accounts Receivable) (Details) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Revenue Recognition [Abstract]    
Customer Accounts Receivable, Billed (Less Reserve) $ 400.0 $ 466.6
Customer Accounts Receivable, Unbilled (Less Reserve) 327.2 $ 346.6
Increase (Decrease) in Customer Accounts Receivable, Billed (Less Reserve) (66.6)  
Increase (Decrease) in Customer Accounts Receivable, Unbilled (Less Reserve) $ (19.4)